Abstract
Cellulose acetate is widely used in films, filters, textiles, lacquer and cosmetic products. Herein we demonstrate the production of cellulose esters under homogeneous conditions using 1,5-diazabicyclo[4.3.0]non-5-ene acetate ([DBNH][OAc]) as solvent. The reagents have been chosen such that the system is recyclable, i.e. by-products are low boiling and easy to remove. It is demonstrated that cellulose acetate can be synthesized with different degree of substitution (DS) values, and that some commonly used acylation regents, like vinyl carboxylates react well without additional base catalyst. Low to high DS values are possible with good recovery of high purity ionic liquid (IL). A linear correlation method of two separate methods, IR and 31P NMR, is proposed to reliably assess the DS of the products. The recyclability of the solvent is demonstrated by acetylating cellulose with isopropenyl acetate to high degree and regeneration into water. After regeneration of cellulose acetate from the IL with addition of water, the residual water was entrained using n-butanol to minimize hydrolysis of [DBNH][OAc], to allow for high recovery and high purity of the ionic liquid. Thus, an overall scheme for batch cellulose acetylation and recovery of [DBNH][OAc] from aqueous solutions is proposed.
Highlights
Cellulose acetate is one of the first synthetic industrial polymers, as the process for cellulose acetate production was patented already in 1894 (Cross et al 1894)
Extensive literature concerning the acetylation of cellulose has been published and more recently there are many articles concerning acetylation in ionic liquids, where cellulose acetate with different degree of substitution (DS) values and uniform distribution of substituents can be obtained directly (Barthel and Heinze 2006; Jogunola et al 2016; Wu et al 2004; Yang et al 2014)
For a sustainable process this would require conversion of a portion of [DBNH][OAc] back to DBN. This is possible through distillation of the recovered ionic liquid (King et al 2011)
Summary
Cellulose acetate is one of the first synthetic industrial polymers, as the process for cellulose acetate production was patented already in 1894 (Cross et al 1894). Several recent articles have assessed the toxicity of different biomass dissolving ionic liquids. Those with shorter anion and cation alkyl chain length are considered to be of lower toxicity (Ruokonen et al 2016; Witos et al 2017). This corresponds with the better capability to dissolve cellulose as cellulose dissolution capability typically decreases as cation or anion volume gets too large (Isik et al 2014)
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